Hi there,
IMHO its not the plugs that cause this problem, possible tdc sensor, possible key/immobiliser fault?
Next time it happens, watch the immobiliser light in the middle of the dash, ignition on, it should go out and the car start, if it flashes or stays on solid you have an immob. fault.
If it goes out, engine spins but not fire up id try a tdc sensor first. We can supply/fit these if needed.
Hope this helps and if you need any more advise dont hesitate to call us!
Mike
Cooksport
07759 437181